HYDRAULIC AND HYDROCHEMICAL SIGNALS FROM TWO ULTRA-DEEP BOREHOLES AT THE KTB TEST SITE, GERMANY

Two deep boreholes were drilled at the continental deep drilling research site (KTB) in central Germany. The main borehole (HB) is 9100 m deep; the pilot hole (VB) reached a depth of 4000 m. Both boreholes were drilled through metamorphic rocks of the Variscian continental collision zone. The upper and lower sections are mainly composed of greywacke-derived paragneiss and a variegated unit of alternating gneiss and metabasite of probably volcano-clastic origin. Metabasite (amphibolite, metagabbro) constitutes the middle section. The openhole of KTB-VB is situated within metabasites. KTB-HB on the other hand has several hydraulic contact zones with different rock types.

During 2002/2003, we carried out a pumping test in KTB-VB of one-year duration and a pumping rate of about one l/s. A total of 23100 m3 of saline water was pumped from the crustal reservoir. During this experiment watertable changes in the KTB-HB, 200 m away, were also monitored. The experiment showed that the water-saturated fracture pore space of the brittle upper crust is highly connected. The pressure-time data from the KTB-VB showed typical hydraulic reaction of a pumped aquifer. Data analysis provided a hydraulic conductivity of the pumped aquifer (3850 – 4000 m depth) of K = 4.05 10-8 m/s. The unexpected high permeability of the fractured continental crust explains the observed hydraulic connection with the 9100 m deep KTB main hole.

The chemical composition of the pumped water remained constant during the 1-year pumping test. TDS is about 62 g/l. The fluid is essentially a binary CaCl2 + NaCl mixture. The Ca/Na ratio reflects the mineralogical composition of the reservoir rock (metagabbros and amphibolites containing Ca-rich plagioclase) and the high salinity results from H2O-loss during water-rock-interaction (alteration).

During 2004/2005, we carried out an injection test in the KTB-VB that lasted several months. The injection rate was about 3.5 l/s. The computed hydraulic conductivity was similar to the one derived from the pumping test. During the experiment, the KTB main hole suddenly started to overflow artesian and the composition of this water suggests shallow crustal inflow points.